In the area of digital printing (the term "printing" is used to encompass both printing and displaying throughout), gray level has been achieved in a number of different ways. The representation of the intensity, i.e., the gray level, of a color by binary displays and printers has been the object of a variety of algorithms. Binary displays and printers are capable of making a mark, usually in the form of a dot, of a given, uniform size and at a specified resolution in marks per unit length, typically dots per inch. It has been common to place the marks according to a variety of geometrical patterns such that a group of marks when seen by the eye gives a rendition of an intermediate color tone between the color of the background (usually white paper stock) and total coverage, or solid density.
Continuous tone images contain an apparent continuum of gray levels. As an approximation to continuous tone images, pictorial imagery has been represented via binary halftone technologies. In order to record or display a halftone image with a scanning system, one picture element of the recording or display surface consists of a j.times.k matrix of sub-elements where j and k are positive integers. A halftone image is reproduced by printing the respective sub-elements or leaving them blank, in other words, by suitably distributing the printed marks.
Halftone image processing algorithms are evaluated in part, by their capability of delivering a complete gray scale at normal viewing distances. The capability of a particular process to reproduce high frequency rendition (fine detail) with high contrast modulation makes that procedure superior to one which reproduces such fine detail with lesser or no output contrast.
Another method of producing gray levels is provided by gray level printing. In such a method, each pixel has the capability to render several different dot sizes. The dot size for a pixel is a function of the exposure time provided an LED element corresponding to that pixel. For example, in some systems the longer the exposure time, the more toner is attracted to that particular pixel.
There are two major concerns in rendering a continuous tone image for printing: (1) the resolution of image details, and (2) the reproduction of gray scales. In a binary halftone representation scheme, these two fundamental factors compete with each other. The more gray levels that are rendered, the larger is the halftone cell. Consequently, coarse halftone line screens are provided, with the attendant poor image appearance. Hence, a compromise is made in rendering between the selection of line resolution and gray scales in binary halftone printing. However, in gray level halftone printing, one can satisfy both resolution and gray level requirements. In gray level printing, the same number of addressable dots are present, and there is a choice of dot sizes from one dot-size of 1 bit/pixel (the binary example) to 16 different dot-sizes of 4 bits/pixel. An image could then be rendered for example with 133 line screens per inch and 128 gray scales of higher quality image. Although providing higher image quality with respect to line resolution and tonal scales, gray level halftoning presents its own dot rendering issues.
A problem exists in the application of a gray level rendering technique to a document that contains different types of images: text, halftone, and continuous tone. These different types of images create different rendering problems, based on a trade-off between tone scales and detail resolution. For example, with text, the number of tone scales is not as important as providing a smooth text edge, whereas the opposite holds true for continuous tone images. Providing a single type of gray level halftone rendering technique to a document that contains two or more types of images may lead to the production of a document in which one or more of the different types of images are reproduced unsatisfactorily.
When scanning a document, image processing techniques have been applied to convert a gray scale image into an image representation which a printer can accept (either binary format or gray level format). In this scanning process, text areas, line drawing and halftone pictures are indistinguishable from each other, and all appear to be a gray scale image. An improper conversion process creates artifacts in the hardcopy such as a jagged boundary in the text area, or a Moire pattern in the halftone region. To overcome this, intelligent processes have been developed to segment the image into different regions of text, line drawing, and picture. Different conversion processes for the individual segments were then applied to these segments to restore the original document. However, these segmentation and conversion processes unduly complicate the digital copying process.
In my U.S. Pat. No. 5,200,831, the contents of which are incorporated herein by this reference, there is described a unified rendering method and apparatus using gray level printing that will satisfactorily reproduce an image that contains text, line drawing, halftone and/or continuous tone regions, with different gray dot representations selected for regions meeting a defined criteria without the need to particularly investigate definitively for specific type of region.
In this patent there is described a method of reproducing an original image comprising the step of scanning an original image to digitize the image so as to produce a digitized image signal, and collecting statistical information of dot regions from the digitized image signal. For each dot region of the digitized image signal, a dot type is selected to render that dot region in accordance with the statistical information, the dot type being either a mixed dot type or a fixed threshold type. A gray level printer is controlled to print the digitized image signal by rendering the dot regions in accordance with the selected dot type for each dot region.
The fixed threshold type dot provides very good rendering of text and halftone, and will not cause Moire in the halftone while producing a smooth text boundary. The mixed dot provides very good rendering for continuous tone regions. By collecting statistical information and then locally selecting the appropriate dot type from between the fixed threshold type and the mixed dot type, a superior reproduction of an image that contains different types of image regions, such as text, halftone and continuous tone regions is achieved.
The fixed threshold dot type is rendered by comparing the gray level of the scanned dot typically represented by an 8-bits per pixel value from 0-255 with each of 15 threshold values (4 bits/pixel printing system) and selecting a gray level representing the closest threshold value to render the dot. A characteristic of the fixed threshold type dot is that each scanned pixel has its corresponding printed gray level determined by the same set of 15 threshold values (see FIG. 8). Thus, 15 levels of gray are realizable at the local pixel level but larger levels of gray are not provided since this type of dot does not contemplate clusters of pixels arranged as cells and the higher numbers of gray levels realizable by such cells albeit at lower resolutions.
The mixed dot type described in the aforementioned patent builds on the capability of a gray level printer to render several dot sizes.
As noted above, in gray level printing, each pixel has the capability to be rendered in several different dot sizes, and thus different gray levels. However, instead of simply providing each pixel with an independent gray level, several pixels may be organized together to form a superpixel, or cell. Each of the pixels in a cell is then provided with a gray level. The human visual response integrates the various gray levels of the individual pixels in the cell to a single perceived gray level for the cell. This is similar to the basic concept of binary halftoning. The number of tone scales for a cell is increased greatly, however, due to the number of different gray levels available for each pixel. For example, instead of only the two levels provided in binary halftoning for each pixel, fifteen levels can be provided with gray level printing for each pixel in a cell (4 bits/pixel). When the cell is made up of 8 pixels, such as outlined cell 10 in FIG. 3, for example, the gray level printing allows 121 different gray shades (including 0) to be rendered for that cell.
The formation of the dots in the pixels of a cell can be performed in a number of different ways to achieve different desired results. The growth pattern for the dots can be formed as "full" dot, "partial" dot, or "mixed" dot to provide gray level halftoning.
The growth pattern of gray levels in cell 10 grows in accordance with that of the so-called mixed dot type. The mixed dot type in-turn is a hybrid of the full dot type and partial dot type growth patterns. In the full dot type growth pattern, increases in cell gray level are rendered by increases in gray level at a single pixel location within the cell until the maximum gray level for that pixel is realized. The next series of higher gray levels for the cell are realized by increases in gray level at an adjacent cell location and so on. Thus, in the full dot type the lower cell gray levels tend to be concentrated near the center of the cell. Compare this with the partial dot type cell growth pattern wherein increases in cell gray level are more distributed throughout the entire cell. Thus, for a partial dot type growth pattern, cell gray level 5 would be rendered by a gray level of one at 5 pixel locations within the cell whereas in the full dot type, a cell gray level of 5 is rendered by one pixel location in the cell having a gray level of 5 and all the others are 0.
In the electrophotographic process, the full dot type formation process is favored because it forms stable dots and exhibits less granularity (halftone printing noise). The partial dot method, however, carries more information detail than full dot, but at the cost of less stable dots.
As noted in the aforementioned patent, the mixed dot type cell combines the merits of both the full dot and the partial dot types in gray level halftoning. A number of different processes can be provided to combine the full dot type and the partial dot type, with the specific mixed dot type being chosen based on which renders an image with more smoothness, less graininess, and more image details. Suggested strategies are: 1) build small stable dots in the highlight (toe) region; 2) keep tone response linear in the mid-tone region; 3) reduce dot structure in the shadow (shoulder) region and render more details. Based on these considerations, a specific mixed dot type is chosen to optimize stable dots, more image detail and less graininess.
An example of a specific mixed dot type 3-bit gray halftone dot layout is illustrated in FIG. 3. As can be seen, until gray level 96 is reached, the pixels are constrained from growing beyond dot-size of 12. The pixels grow in a full dot type process, with the pixel circled growing to a dot size of 12 for cell gray levels 1 through 12. For higher cell gray levels the pixel location 30 that is squared then starts to grow in size. Note that this is a 4-bits/pixel system and that the maximum density or dot size at a pixel location is 15. Thus, when the cell gray level is to be 12, it is rendered by a pixel of size or density 12 at the circled location. However, a cell gray level of 13 is rendered by a pixel of density 12 at the circled location and a pixel of density one at the squared location. Once all of the pixels in the cell have attained a dot size of 12, corresponding to cell gray level 96, the cell then increases in gray level by using a partial dot type process. In other words, each of the pixels in the cell must grow to a dot size of 12 before any of the pixels begins growing to a dot size of 13. Thereafter, increases in cell gray level are incrementally and orderly dispersed through the cell. For example, cell grey level 98 results from two pixels in the cell being grey level 13 and 6 pixels being grey level 12.
As noted above and with reference to FIG. 8, another type of rendering technique is the fixed threshold method. In this method each individual pixel is rendered with only limited tone scales. For example, 4 bits/pixel renders 16 different tone shades, including 0. The fixed threshold type renders the highest resolution among the various types, and an edge can be rendered more accurately down to each pixel. The fixed threshold type renders an image with even higher sharpness than the partial dot type since it is not limited by the cell size as is the partial dot type. The problem with the fixed threshold type is that it has less tone scales, so that a false contour could easily be seen in the rendered image. However, the fixed threshold type will provide excellent rendering results on text and halftone originals.
Although any one of the three dot types (full, partial or mixed) could be used to produce a satisfactory continuous tone image, the mixed dot type is the best choice for continuous tone rendering. As stated above, the fixed threshold type renders well on both text and halftone. The unified rendering technique of the invention disclosed in the aforementioned patent uses both fixed threshold type and mixed dot types according to local image content so that text, halftone and continuous tone images are all reproduced well.